1. FIELD OF THE INVENTION This invention relates to an improved process and apparatus for producing sodium hydrosulfite.
Sodium hydrosulfite, Na.sub.2 S.sub.2 O.sub.4, also known as sodium dithionite, is extensively used as a bleaching agent in the paper and textile industries, and has a wide range of other uses. Because it is relatively unstable, it is generally produced in situ at the point of use, for example in a pulp mill.
2. DESCRIPTION OF THE PRIOR ART
Methods used in the past for producing sodium hydrosulfite have included dissolving zinc in a solution of sodium bisulfite and precipitating the zinc-sodium sulfite with milk of lime to leave the hydrosulfite in solution, and reacting sodium formate with sodium hydroxide and sulfur dioxide.
More recently the present applicants have developed a process wherein caustic soda and sulfur dioxide are mixed with sodium borohydride in an aqueous medium to produce an aqueous solution of the hydrosulfite. The sodium borohydride generally enters the process in a mixture with aqueous sodium hydroxide. This mixture obtainable from the Ventron Division of Morton Thiokol Inc., under the registered Trade Mark BOROL, has good stability since acid hydrolysis of the sodium borohydride is prevented. For convenience, this type of process will be referred to hereinafter as the Borol process.
The theoretical reaction of the Borol process, assuming ideal conditions and 100% yield, would be as follows: EQU NaBH.sub.4 +8NaOH+8SO.sub.2 .fwdarw.4Na.sub.2 S.sub.2 O.sub.4 +NaBO.sub.2 +6H.sub.2 O
There is however a side reaction in which the sodium borohydride is hydrolysed, thus reducing the overall efficiency of the reaction: ##STR1##
This reaction is a function of pH, and increases with reduced pH. The problem cannot however be overcome simply by raising the pH since this would adversely affect the main reaction. The reaction effectively takes place in two stages, as follows:
(a) the reaction between sulfur dioxide and caustic soda to give sodium bisulfite (I) and
(b) the reaction between the bisulfite and sodium borohydride to give sodium hydrosulfite (II). EQU (I) 8NaOH+8SO.sub.2 .fwdarw.8NaHSO.sub.3 EQU (II) 8NaHSO.sub.3 +NaBH.sub.4 .fwdarw.4Na.sub.2 S.sub.2 O.sub.4 +NaBO.sub.2 +6H.sub.2 O
There is also an equilibrium (III) between the bisulfite and sodium sulfite, which is a function of the pH: ##STR2## K.sub.2 =1.02.times.10.sup.-7 (18.degree. C.) K.sub.1 =1.54.times.10.sup.-2 (18.degree. C.)
Above pH.sup.7, the bisulfite concentration is inversely proportional to pH. Below pH2, the bisulfite concentration is directly proportional to pH. In the pH range 5-7, within which this type of process is generally operated, lowering the pH will favour the formation of bisulfite.
Consideration of this equilibrium therefore has to be weighed against that of acid hydrolysis discussed above to determine the optimum pH for the process. In the process used hitherto a pH of 6.5 has been found to give the best yield. Nevertheless, it has proved difficult to achieve yields greater than 85%.
In the Borol process used hitherto, SO.sub.2, water, sodium hydroxide and a sodium borohydride/sodium hydroxide/water mixture (Borol) are fed in that order into a flow line which leads to a static mixer and thence to a degassing tank where entrained gases are vented to the atmosphere. An aqueous solution of sodium hydrosulfite is pumped from the degassing tank, part of this being delivered to a storage tank for use as required and the rest is recycled to the flow line at a position downstream of the SO.sub.2, water and NaOH inlets but upstream of the Borol inlet. The input of each reactant can be controlled automatically in response to rising or falling levels in the degassing tank or the storage tank or changes in pressure, flow rates and/or pH.
The present applicants have sought to improve the yields of the above process by variation in proportions of chemicals, pH measurement and control and different addition methods.